Receiver Having Limiter-Enhanced Data Eye Openings

ABSTRACT

A communication system having a receiver with a linear path and a nonlinear path. As the receiver receives a data signal, it adaptively equalizes the received signal, and amplitude-limits the equalized signal in the nonlinear path using a saturable amplifier limiter or the like. A slicer extracts data from the limited equalized received signal. In the linear path, a clock recovery circuit generates a clock signal from the equalized received signal. A delay circuit in the linear path at least partially compensates for propagation delay in the limiter. Having the clock recovery occur in other than the nonlinear path, a low jitter clock is generated. The limiter enhances the vertical opening of the data eye by increasing the rise and fall times of the limited signal, providing more noise margin for the slicer to operate with and a greater timing margin in which to sample the sliced data.

BACKGROUND

Multi-gigabit per second (Gbps) communication between various chips or“ports” on a circuit board or modules on a backplane has been in use forquite a while. Data transmission is usually from a transmitter thatserializes parallel data for transmission over a communication media,such as twisted pair conductors as a cable or embedded in a backplane,fiber optic cable, or coaxial cable(s), to a receiver that recovers thetransmitted data and deserializes the data into parallel form. However,data transmission greater than 20 Gbps over communication paths has beendifficult to achieve because various signal impairments, such asintersymbol interference (ISI), crosstalk, echo, and other noise, cancorrupt the received data signal to such an extent that a receiverhaving various decision feedback and analog equalizers might not be ableto recover the transmitted data at the desired high data rate with anacceptable level of error performance. One possible reason for thisshortcoming is that data rates 20 Gbps and higher are approaching themaximum speed of the transistors used to implement the receiver,reducing the amount of signal processing that can be achieved to improvethe quality (e.g., the amount of “eye opening”) of the received signal.Further, because power consumed by CMOS circuitry is generallyproportional to the frequency of signals being processed by thecircuitry, the amount of power consumed by a receiver operating at thesedata rates might be unacceptably high when the receiver is providing theacceptable level of error performance.

One way to improve the quality of the received signal is for the signaltransmitter, operating in conjunction with the signal processingcircuitry in the receiver, to drive the channel with signals that havebeen pre-distorted by a filter. One such filter used to pre-distort thetransmitted signal is a finite-impulse response (FIR) filter withadjustable coefficients or taps, referred to herein as a TXFIR filter.With most common data communication standards (e.g., PCIe Gen3, SAS-3,16GFC, and 10 GBASE-KR, all of which are included herein by reference intheir entirety), the coefficients of the TXFIR filter are controlled bythe receiver using a back channel from the receiver to the transmitter.However, as with receivers, the high data rates are approaching thespeed of the transistors in the transmitter, thereby reducing the amountof pre-distortion the TXFIR can provide. Further, as a consequence ofusing as high a speed transistors as possible in the transmitter, thevoltage handling capability of he transistors is concomitantly reducedso that the amplitude of the data signals from the transmitter arereduced and further degrading the quality of the signals at thereceiver.

Therefore, it is desirable to provide a receiver that can readilyprocess 20 Gbps or higher data rates at an acceptable level of powerconsumption.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tolimit the scope of the claimed subject matter.

One embodiment of the invention is a receiver having an input andadapted to receive a signal from a communication channel. The receivercomprises an equalizer, a limiter, a decision circuit, and a clockrecovery circuit. The equalizer is coupled to the input of the receiverand the limiter is coupled to an output of the equalizer. The decisioncircuit, having a clock input, is coupled to an output of the limiter.The clock recovery circuit, coupled to an output of the equalizer andnot connected to the output of the limiter, generates a clock signalthat is coupled to the clock input of the decision circuit.

In another embodiment of the invention, a method of receiving a datasignal in a receiver having a linear path and a nonlinear path,comprises the steps of recovering, from the nonlinear path, data fromthe received data signal; and recovering, from the linear path, a clocksynchronized to the received data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will become more fully apparent from thefollowing detailed description, the appended claims, and theaccompanying drawings in which like reference numerals identify similaror identical elements.

FIG. 1 is a simplified block diagram of a communication system having atransmitter and a receiver communicating over a communication channel aswell as a back channel according to one embodiment of the invention;

FIGS. 2 and 3 are simplified eye diagrams of received baseband signals;

FIG. 4 illustrate the effects of an ideal limiter on eye openings; and

FIG. 5 is a simplified flowchart illustrating initialization of thecommunication system shown in FIG. 1 according to an embodiment of theinvention.

DETAILED DESCRIPTION

In addition to the patents referred to herein, each of the followingpatents and patent applications are incorporated herein by reference intheir entirety:

-   -   U.S. Pat. No. 7,599,461, titled “Method and Apparatus for        Generating One or More Clock Signals for a Decision-Feedback        Equalizer Using DFE Detected Data in the Presence of an Adverse        Pattern”, by Aziz et al.    -   U.S. patent application Ser. No. 12/776,681, titled “A        Compensated Phase Detector for Generating One or More Clock        Signals Using DFE Detected Data in a Receiver”, by Aziz et al.    -   U.S. Pat. No. 8,040,984, titled “Methods and Apparatus for        Improved Jitter Tolerance in an SFP Limit Amplified Signal”, by        Pervez Aziz et al.    -   U.S. patent application Ser. No. 13/360,978, filed 30 Jan. 2012,        titled “Transmitter Adaptation Loop Using Adjustable Gain and        Convergence Detection”, by Mohammad Mobin et al.    -   U.S. patent application Ser. No. 11/541,379, filed 29 Sep. 2006,        titled “Method and Apparatus for Non-Linear Decision-Feedback        Equalization In the Presence of Asymmetric Channel”, by Mohammad        Mobin et al.    -   U.S. patent application Ser. No. 12/493,435, filed 29 Jun. 2009,        titled “Real-Time Eye Monitor for Statistical Filter Parameter        Calibration”, by Mohammad Mobin et al.    -   U.S. patent application Ser. No. 11/86.4,110, filed 28 Sep.        2007, titled “Methods and Apparatus for Determining Threshold of        One or More DFE Transition Latches Based on Incoming Data Eye”,        by Pervez Aziz et al.    -   U.S. Pat. No. 7,711,043, filed 29 Sep. 2006, titled “Method and        Apparatus for Determining Latch Position for Decision-Feedback        Equalization Using Single-Sided Eye”, by Mohammad Mobin et al.

For purposes of this description and unless explicitly stated otherwise,each numerical value and range should be interpreted as beingapproximate as if the word “about” or “approximately” preceded the valueof the value or range. The term “or” should be interpreted as inclusiveunless stated otherwise. Further, signals and corresponding nodes,ports, inputs, or outputs may be referred to by the same name and areinterchangeable. Additionally, reference herein to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment can beincluded in at least one embodiment of the invention. The appearances ofthe phrase “in one embodiment” in various places in the specificationare not necessarily all referring to the same embodiment, nor areseparate or alternative embodiments necessarily mutually exclusive ofother embodiments. The same applies to the terms “implementation” and“example”.

Also for purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect”, “connecting”, or “connected” refer to any mannerknown in the art or later developed in which a signal is allowed to betransferred between two or more elements and the interposition of one ormore additional elements is contemplated, although not required.Conversely, the terms “directly coupled”, “directly connected”, etc.,imply the absence of such additional elements.

As data rates increase for serializer/deserializer (SERDES)applications, the channel quality degrades and the use of decisionfeedback equalization (DFE) in conjunction with an finite impulseresponse (FIR) filter in a transmitter (TX) and an analog equalizerwithin a receiver (RX) is generally used to achieve the bit error rate(BER) performance needed for reliable communications.

A digital receiver, such as the receiver 10 shown in FIG. 1 andexplained in more detail below, recovers digital signals by samplingbaseband analog waveform (the received signal) and then reliably detectthe sampled data. Signals arriving at a receiver are typically corruptedby intersymbol interference (ISI), crosstalk, echo, and other noise. Asdata rates increase, the receiver must both equalize the channel, tocompensate for such corruptions, and detect the encoded signals atincreasingly higher clock rates. Decision-feedback equalization (DFE) isa widely used technique for removing intersymbol interference and othernoise at high data rates. For a detailed discussion of decision feedbackequalizers, see, for example, Digital Communication Principles by R.Gitlin et al (Plenum Press 1992) and Digital Communications by E. A. Leeand D. G. Messerschmitt (Kluwer Academic Press, 1988), each incorporatedby reference herein in their entirety.

Generally, decision-feedback equalization utilizes a multi-tap equalizerto equalize the channel using a feedback loop based on previouslydetected (or decided) data. In one typical DFE implementation, areceived analog signal is sampled after DFE correction and compared toone or more first thresholds to generate the detected data and one ormore second thresholds usually hut not necessarily different from thefirst) to generate adaption error signals as will be described below.The DFE correction is subtracted in a feedback fashion to produce aDFE-corrected signal. A clock, generated from the received signal by aClock Recovery (CR) circuit, is generally used to sample theDFE-corrected signal and for the DFE operation. An example of such areceiver is disclosed in “Method and Apparatus for Generating One orMore Clock Signals for a Decision-Feedback Equalizer Using DFE DetectedData”, by Aziz et al, U.S. Pat. No. 7,616,686, incorporated by referenceherein in its entirety. The receiver described in the patent utilizes aDFE-based phase detection architecture for clock and data recovery of aDFE-equalized signal.

A DFE-based receiver might include an analog front end (AFE), typicallyhaving an adjustable gain amplifier (AGA) used to control the inputsignal level, and an equalizer used to compensate for frequency-baseddistortions in the input signal to the receiver. The analog equalizer(also referred to herein as a continuous-time linear equalizer or CTLE)in the AFE is generally implemented as an analog-based filter with atleast one adjustable coefficient or peaking parameter that can at leastpartially compensate for frequency-dependent distortions in the receivedsignal. For example, high frequency portions of the received signalsmight be attenuated as the signals pass though a transmission line,rounding off what are otherwise sharp, square-wave signals as originallytransmitted. The CTLE's coefficients or peaking parameters, in responseto a controller within the receiver, are adjusted to add gain to thehigh frequency components of the signal, referred to herein as“peaking”, to at least partially restore the received signal to itsoriginal shape. However, the analog circuitry in the AFE might haveinherent limitations, one of which is the amount of nonlinear distortionintroduced onto the received signal by various analog circuits in theAFE. Any distortion introduced by the AFE can seriously degradeperformance of the overall receiver.

As data rates increase for serializer/deserializer (SERDES)communications applications, the “quality” of the channel media degradesdramatically even over short distances between the ends of thecommunication channel. One technique typically used in conjunction withthe receiver's DFE and analog equalizers to achieve the desired channelperformance needed for reliable communications over the degraded channelis to pre-distort the transmitted signal to counteract the effects ofthe channel on the signal presented to the receiver. For high-speedsignaling applications, such as 8 Gbps and faster SERDES applications,the pre-distortion characteristics are adjusted through a back orreverse channel to adapt the pre-distortion to the channel'scharacteristics.

FIG. 1 is a block diagram of a typical SERDES communication channel 100having a receiver 10 and a transmitter 20, the transmitter having anequalizer and the receiver having an analog equalizer and a DFE-basedequalizer in tandem. As shown in FIG. 1, baseband data from thetransmitter 20 is transmitted through a backplane communications channel120 after optionally being equalized or filtered through a transmit FIRfilter (TXFIR) 110. After passing through the backplane (BKPLN) 120,metal traces in a substrate (not shown), a cable (not shown), or acombination thereof, the analog data signal received by the receiver 10is processed by equalizer 130. The received signal may optionally befiltered or equalized by an analog front end (AFE) 132 having a variablegain amplifier (not shown) for amplitude control and may include, forexample, a continuous-time filter or the equivalent. The analog signaloutput r_(k) of the AFE 132 passes through subtractor 134, used inconjunction with a decision feedback equalizer (DFE) 136 having L tapsand well understood in the art, to produce an output signal y_(k). Theoutput signal y_(k) is delayed by a variable delay circuit 170 and thedelayed signal on node 175 is then sampled by a clock recovery (CR)circuit 150. A limiter 155, described in more detail below, amplifiesthe signal y_(k) and limits the amplified signal on node 157 to a givenamplitude centered about a midpoint value, in this example,approximately zero although this value is not critical and can beadjusted as desired but is generally determined by the circuitimplementation of the limiter, e.g., a high gain amplifier that isdriven into saturation by the signal y_(k). A slicer 160 (describedbelow) digitizes the output of the limiter 155 by comparing the signalfrom the limiter to a threshold value and latches the result in responseto the data clock generated by the CR 150. In one embodiment, thethreshold of the slicer 160 is approximately the same as the midpointvalue of the signal from the limiter 155.

In one exemplary embodiment, the limiter 155 is a conventional amplifier(not shown) having enough gain for it to saturate when amplifying theDFE-corrected signal y_(k). The saturation of the amplifier will limitor clamp the amplified signals to voltage levels related to power supplyvoltages coupled to the limiter 155. A saturated amplifier might exhibita propagation delay that results from the amplifier “switching” from oneoutput level to the other in response to a change in the input signalthereto and may be several picoseconds long, a significant portion ofthe received data's bit period. To reduce or control the amount ofpropagation delay, the amplifier might have anti-saturation circuitryadded thereto as is known in the art. To compensate for the propagationdelay of the limiter 155, the variable delay circuit 170 is provided totime-align the DFE-corrected analog signal y_(k) with the limited signalon node 157, as described in more detail below.

A conventional slicer 160 might be implemented using one or moreslicer-latches (i.e., a decision device based on an amplitude thresholdand a latch to hold the results of the decision device) or a morecomplicated detector such as a sequence detector. For high-speedapplications, the slicer 160 is often implemented as two slicer-latches162, 164 that each sample the limited signal on node 157 in response toa locally generated sample clock, here sample clock C_(m) from CR 150.In addition to sampling the data signal, each slicer-latch in slicer 160essentially quantizes the signal to a binary “1” or “0” based on theanalog value of the signal applied to the slicer, represented here as s,and a slicer threshold, s_(t). If the input to the slicer 160 at time kis s_(k), then the detected data bit output â_(k) of the slicer 160 isgiven as follows:

$\begin{matrix}{{\hat{a}}_{k} = {{1\mspace{14mu} {if}\mspace{14mu} s_{k}} > s_{t}}} \\{= {0\mspace{14mu} {{otherwise}.}}}\end{matrix}$

In this example, the slicer-latch 162 has a slicer threshold s_(t) andthe slicer-latch 164 has a slicer threshold −s_(t), where s_(t) is theresidual post-cursor tap value of the DFE 136. Alternatively, the slicerthreshold values are a fixed value, e.g., zero. The DFE 136 and thepost-cursor tap values of the DFE are well known in the art.

The phase of the analog input signal is typically unknown and there maybe a frequency offset between the frequency at which the original datawas transmitted and the nominal receiver sampling clock frequency. Thefunction of the CR 150 is to generate a clock that allows for the propersampling of the limited analog signal from limiter 155 such that thedata is recovered properly when the signal on node 157 is sampled byslicer 160. The CR 150 may be conventional and is often an adaptivefeedback circuit where a feedback loop adjusts the phase and frequencyof the nominal clock to produce the modified recovered clock C_(m) thatis used by slicer 160 sample the signal from limiter 155 in the centerof the eye opening in the signal to allow proper data detection withmargins, as will be illustrated in connection with FIG. 3. In addition,the CR 150 provides a recovered clock signal for use by a utilizationdevice or data sink (not shown) that utilizes the detected data bitsâ_(k), as well as an error detector 180 and the DFE 136.

In general, the CR 150 may be composed of several components, such as aphase detector, a loop filter, and a clock generation circuit (notshown). In one embodiment, the CR 150 comprises a bang-bang phasedetector (BBPD). For a general discussion of bang-bang phase detectors,see, for example, J. D. H. Alexander, “Clock Recovery from Random BinarySignals,” Electronics Letters, 541-42 (October, 1975), incorporated byreference herein in its entirety. Alternatively, the CR 150 comprises aMueller-Muller CR where the signals are sampled at the baud-rate. For ageneral discussion of Mueller-Muller CR, see, for example, K. Muellerand K. Muller, “Timing Recovery in Digital Synchronous Data Receivers,”IEEE Trans. Conlin., Vol. 24, No. 5, May 1976, pp. 516-531, incorporatedby reference herein in its entirety.

In one embodiment, a conventional error detector 180 has two samplinglatches (not shown) that, in response to the sampling clock C_(m),samples the time-aligned DFE-corrected signal on node 175 using twodifferent slicing levels to generate an error signal e_(k). The errorsignal is used as part of a feedback mechanism to adjust thecoefficients of DFE filter 136 via path 140, AFE 132 (if provided) viapath 142, and the TXFIR 110 (if provided) via path 144, therebyimproving the performance of the system 100. The slicing levels of thesampling latches in the error detector 180 are set to be approximatelyequal to the magnitude of the primary cursor tap value in the DFE 136.The DFE 136 and the primary cursor tap value of the DFE are well knownin the art.

A conventional adaptation controller 138, responsive to the error signale_(k) and the detected data bits â_(k), generates the coefficientcontrol signals for adjusting the one or more coefficient values of theL-tap DFE 136 and the AFE 132. During a training or adaptation phase bythe receiver 10, the controller 138, responsive to the detected databits â_(k) and the error signal e_(k), converges one or more coefficientvalues of the AFE 132 to reduce intersymbol interference during eyeopenings or to reduce signal transition jitter. An exemplary techniquefor adapting the AFE is described in “Adaptation Algorithms for a Classof Continuous Time Analog Equalizers With Application to Serial Links”,by P. M. Aziz and A V. Malipatil, Proceedings of the 2011 IEEEInternational Symposium on Circuits and Systems (ISCAS 2011), pp.1383-1386, included by reference herein in its entirely. Similarly,operation and adaptation of L-tap DFE 136 is well known and explanationof the filter 136 and alternative embodiments thereof may be found inthe above-referenced book Digital Communication Principles by R. Gitlinet al. For purposes here, the controller 138, in response to the errorsignal e_(k) from the error detector 180 and the detected data bitsâ_(k) from slicer 160, adjusts the filter coefficient values for the DFEfilter 136 at least during the above-mentioned training or adaptationphase by the receiver. The controller 138 uses the well-knownleast-mean-square algorithm or the like to minimize or reduce below athreshold value the mean-square of the error signal e_(k). Once thevalues of the DFE coefficients or AFT coefficients stabilize orconverge, a DFE correction signal z_(k) is generated by a DFE filter 136from the detected data bits â_(k) and the correction signal issubtracted by the analog summer 134 from the output r_(k) of the AFE 132to produce the DFE-corrected signal y_(k), where y_(k)=r_(k)−z_(k). Thenthe DFE-corrected signal y_(k) is limited by limiter 155 then detectedby the slicer 160 to produce the detected data bits â_(k).

As mentioned above, the threshold values for the slicer 160 and samplinglatches in the error detector 180 might be determined by the tap valuesof the DFE 136 as set by the adaption controller 138. Alternatively, thethreshold values might be determined by feedback mechanisms such as thatdescribed in the aforementioned U.S. patent application Ser. No.11/864,110 and U.S. Pat. No. 7,711,043.

In addition to the adaptation of the coefficients of the DFE 136 and AFE132, the controller 138 might also adjust the coefficients of the TXFIRfitter 110 via path 144. As mentioned above, data to be transmitted isfiltered through the TXFIR filter 110 disposed between a serializer andthe communication channel media to improve the performance of the system100 by pre-distorting the signals applied to the conductors in thechannel media. An exemplary implementation and technique for adaptingthe TXFIR is described in the above-identified U.S. patent applicationSer. No. 13/360,978, filed 30 Jan. 2012, titled “Transmitter AdaptationLoop Using Adjustable Gain and Convergence Detection”, by Mohammad Mobinet at. It is understood that alternative methods and embodiments may beused to implement the TXFIR and the control thereof.

FIG. 2 illustrates simulated representative traces of the DFE-correctedsignal y_(k) after the DFE, AFE, and TXFIR coefficients have stabilized,i.e., converged, but without the benefit of using a limiter. This imageillustrates that the eye opening might have too slow a rise/fall timeand is distorted, even with equalization, to make accurate placement ofthe slicer 160's thresholds (vertical axis) and sample clock phase(horizontal axis) in approximately the middle of the eye openingproblematic without using potentially complex and power-hungry adaptiveeye placement and DFE adaptation techniques, such as that described inabove-identified U.S. patent application Ser. No. 11/541,379, that mightnot be fast enough for 20 Gbps data rates. In FIG. 2, signal traces for1→x and 0→x data transitions in the DFE-corrected signal y_(k) are shownwith different shading. When viewing the DFE-corrected signal as asuperposition of the two sets of data transitions, the size of the dataeye opening is approximately associated with the inner circle 270,having an associated timing margin 230 (horizontal axis) and noisemargin 260 (vertical axis). The small size of the data eye 270 with thecorresponding poor margins 230 and 260 makes it very difficult to placesample point 272 to properly recover the transmitted data. The size ofthe data eye 270 is constrained because of remaining distortion in the1→x data transitions 210 and the 0→x data transitions 220 even thoughthe DFE and AFE have both converged. When the received signal is viewedto only have the 1→x transitions 210, the distortion that would normallybe associated with the 0→x transitions is absent and the resulting upperDFE eye, approximately associated with the circle 280, is larger thanthe small data eye 270 associated with both sets of transitions. Theupper DFE-corrected eye 280 has an associated timing margin 240 andnoise margin 250. Likewise, when the data is viewed to only have 0→xtransitions 220, the distortion that would normally be associated withthe 1→x transitions is absent and the resulting lower DFE eye is largerthan the small data eye associated with both data transitions present.

Using sample point 282 and corresponding sample point 284 (positioned tohave approximately the same phase as the sample point 282 but with thethreshold value of −s_(t)) to sample the DFE-corrected signal y_(k),there is an improvement in performance of the receiver but the phase ofthe sample clock is advanced from that of sample point 272 and mightstill have margin problems that limit the performance of the receiver.Moreover, the adjustment of the sample clock can be difficult andprocessing intensive, such as that disclosed in the aforementioned U.S.patent application Ser. No. 11/541,379.

One effect of the limiter 155 (FIG. 1) on the DFE-corrected signal y_(k)shown in FIG. 2 is to increase the height (along the vertical axis) ofeye opening as illustrated in the signal traces shown in FIG. 3. Thevertical opening of the eye 310 is now at least as large as thedifference between the extremes of the eye openings for the individual0→x and 1→x transitions, illustrated by noise margin 290 and 390 inFIGS. 2 and 3, respectively. Another effect of the limiter 155 is thatthe data eye 310 of the amplitude-limited signal (here the signal onnode 157 in FIG. 1) shown in FIG. 3 has more symmetry than that of thesignal shown in FIG. 2, giving a greater timing (horizontal) margin.Because the eye has fast rise and fall times, the center of thehorizontal opening of the eye does not change significantly with thevalue of the slicer threshold s_(t), making the horizontal positioningof the sample points 282, 284 relatively insensitive to the value ofs_(t). The larger data eye allows for the placement of the slicerthresholds s_(t), −s_(t) (illustrated here as levels 365A and 365B,corresponding to slicer levels s_(t) and s_(t) for slicer-latches 162,164 in slicer 160 of FIG. 1) and position or phase of the sample clock,C_(m) which causes the slicer-latches 162, 164 to sample at samplepoints 382, 384 the limited signal applied thereto in approximately themiddle of the data eye 310, to have more margin, and thus to be lesscritical, than that required to correctly sample the signal shown inFIG. 2.

The height, width, and symmetry of the eye opening in FIG. 3 is alsodependent on how “hard” the limiting action is and the amount of gainprovided by the limiter 155. The harder the limiter, i.e., the amount ofcompression the limiter provides once the limiter saturates (typicallymeasured in the amount of change in the output amplitude for a givenchange in input amplitude), the less variation in the amplitude(vertical axis) of the eye opening. All else being equal, the higher thegain of the limiter the larger the vertical opening the eye opening is.As illustrated in FIG. 4, a perfect limiter has infinite gain and novariation in the amplitudes of the output signal to produce, forexample, a perfect “square wave” having variable width pulses withamplitudes limited by the voltage of the limiter's power supply. Becauseof the essentially rectangular opening of the data eye 410, samplepoints 482, 484 (corresponding to the slicing thresholds s_(t), −s_(t)of slicer 160) is simply placed in the center of the data eye 410. Thereis no need to adjust the phase of the sampling clock C_(m) to move thesample point 482, 484 either earlier or later in time.

A practical limiter, such as that used to produce the signal traces inFIG. 3, has high but finite gain resulting in an output signal having apeak amplitude that varies somewhat with the amplitude of the inputsignal. While the use of a less than ideal limiter 155 will result asmaller eye opening with less steep transitions due to slower rise andfall times, an increase in the opening vertically will resultnonetheless, making placement of the slicing thresholds s_(t), −s_(t) ofslicer 160 relatively less critical. In one exemplary embodiment thegain of the limiter is approximately 6 to 12 dB. Moreover, the phase ofthe sample clock C_(m) can be fixed in the center of the eye 310 (FIG.3) but may also be either advanced or retarded by an amount as needed toprovide additional timing margin but without the need for adaptive clockadjustment as taught in the aforementioned U.S. patent application Ser.No. 11/541,379. While the operation of the slicer 160 might be lesscritical when using the limiter 155, deriving the clock from theamplitude-limited signal on node 157 might result in the sample clockC_(m) and the recovered clock having too much jitter for someapplications. To reduce the amount of jitter in the various clocksignals, the CR 150 derives timing information from the time-alignedDFE-corrected signal on node 175, referred to as the “linear path”,instead of from the amplitude-limited signal on node 157, referred toherein as the “nonlinear path”.

As mentioned above, the delay 170 is used to time-align theDFE-corrected signal y_(k) from the subtractor 134 with the outputsignal from the limiter 155. A controller (not shown but might beimplemented by the controller 138) adjusts the amount of delay providedby the delay circuit 170 such that the signals on node 175 areapproximately time-aligned with the signals on node 157. Generally, thecalibration of the delay circuit 170 occurs after the AFE 132 and theDFE 136 have converged.

In an alternative embodiment, the delay circuit 170 is merged with theCR 150 such that the CR 150 is configured to adjust the phase of theclock C_(m) as required, as is well known in the phase-locked ordelay-locked loop art.

In FIG. 5, an exemplary initialization process 500 of the system 100(FIG. 1) is shown. In this exemplary process, the coefficients in theTXFIR 110, AFE 130, and DFE 136 (FIG. 1) are set or initialized to aknown value in step 502. Then a training sequence, e.g., a randomizeddata pattern, is sent as data applied to the TXFIR filter 110 in step504. In response, the controller 138 in step 506 begins the adaptationof the filter coefficients in the AFE 130 and DFE 136 eitherconcurrently or sequentially and repeats step 506 until in step 508 thecoefficients have been determined to converge. Then the coefficients inthe TXFIR 110 are adjusted until converged in steps 510 and 512. It isunderstood that the convergence of the TXFIR coefficients might be doneconcurrently with that of the AFE and DFE. Next, in step 514, thevariable delay circuit 170 is adjusted until the signals on nodes 157and 175 are approximately aligned. Lastly, in step 516, the trainingsequence is terminated and system 100 begins normal operation.

Generally, the CR 150 is enabled during the entire initializationprocess 500 so that it locks onto the delayed signal on node 175 togenerate the sampling clock C_(m) and the recovered clock.Alternatively, the CR 150 may be enabled in step 502 and subsequentsteps are not performed until the CR 150 has achieved lock.

It is understood that various changes in the details, materials, andarrangements of the parts which have been described and illustrated inorder to explain the nature of this invention may be made by thoseskilled in the art without departing from the scope of the invention asexpressed in the following claims.

Although the elements in the following method claims, if any, arerecited in a particular sequence with corresponding labeling, unless theclaim recitations otherwise imply a particular sequence for implementingsome or all of those elements, those elements are not necessarilyintended to be limited to being implemented in that particular sequence.

1. A receiver having, an input and adapted to receive a signal from acommunication channel, comprising: an equalizer, coupled to the input ofthe receiver, having an output; a limiter, coupled to the output of theequalizer, having an output; a variable delay circuit, coupled to theoutput of the equalizer, having, an output; a decision circuit, coupledto the output of the limiter, having an output and a clock input; and aclock recovery circuit, coupled to the output of the equalizer and notconnected to the output of the limiter, for generating a clock signalcoupled to the dock input of the decision circuit.
 2. The receiver ofclaim 1 further comprising: an adaption circuit, coupled to the outputof the decision circuit and the output of the equalizer; wherein theequalizer is an adaptive equalizer having coefficient values controlledby the adaptation circuit and the decision circuit has at least onethreshold value controlled by the adaptation circuit.
 3. The receiver ofclaim 2 wherein the adaptation circuit comprises: an error detectioncircuit, coupled to the output of the decision circuit and the output ofthe adaptive equalizer, for producing an error signal; and an adaptationcontroller, coupled to the output of the error detector circuit and theoutput of the decision circuit, is adapted to implement an adaptationalgorithm for reducing the error signal by controlling the coefficientsof the adaptive equalizer.
 4. The receiver of claim 3 wherein theadaptive equalizer comprises a decision feedback equalizer havingmultiple tap coefficients controlled by the adaptation controller, theat least one threshold value being substantially equal to one of the tapcoefficients.
 5. The receiver of claim 3 wherein the adaptationalgorithm is a least-mean-square algorithm for reducing the mean-squareof the error signal.
 6. The receiver of claim 3 further comprising ahack channel for communicating transmit filter coefficient valuesgenerated by the adaptation controller to a transmitter coupled to thecommunications channel.
 7. The receiver of claim 1 wherein the signal isa baseband digital signal and the communication channel is a backplane.8. The receiver of claim 1 wherein the limiter is a saturable amplifier.9. The receiver of claim 1 further comprising a variable delay circuit,coupled to the output of the equalizer, having an output; wherein theclock recovery circuit is coupled to the output of the variable delaycircuit, the limiter has a propagation delay, and the variable delaycircuit provides a delay for at least partially compensating for thepropagation delay of the limiter.
 10. The receiver of claim 1 whereinthe clock recovery circuit is coupled to the output of the equalizer,the limiter has a propagation delay, and the clock recover circuit isadapted at least partially compensating for the propagation delay of thelimiter.
 11. The receiver of claim 10 further comprising: an errordetection circuit, coupled to the output of the decision circuit and theoutput of the variable delay circuit, having an output; and an adaptioncontroller, coupled to the output of the decision circuit and the outputof the error detection circuit; wherein the equalizer is a multi-tapadaptive equalizer having tap coefficient values controlled by theadaptation controller.
 12. A receiver having an input and adapted toreceive a signal from a communication channel, comprising: an analogfront end, coupled to the input of the receiver, having an output and atleast one filter coefficient; an adaptive equalizer, coupled to theoutput of the analog front end, having an output and tap coefficientvalues; a limiter, coupled to the output of the equalizer, having anoutput; a decision circuit, coupled to the output of the limiter, havingan output and a clock input and at least one threshold value; a variabledelay circuit, coupled to the output of the equalizer, having an output;an error detection circuit, coupled to the output of the decisioncircuit and the output of the variable delay circuit, for producing anerror signal; an adaption controller, coupled to the output of thedecision circuit and responsive to the error signal, adapted toimplement a least-mean-squared algorithm for reducing the error signalby controlling the tap coefficient values of the adaptive equalizer, theat least one threshold value of the decision circuit, and the at leastone filter coefficient of the analog front end; and a clock recoverycircuit, coupled to the output of the equalizer and not connected to theoutput of the limiter, for generating a clock signal coupled to theclock input of the decision circuit; wherein the clock recovery circuitis coupled to the output of the variable delay circuit, the limiter hasa propagation delay, the variable delay circuit provides a delay for atleast partially compensating for the propagation delay, and thethreshold value is set to approximately an average value of an outputsignal on the output of the limiter.
 13. The receiver of claim 12further comprising a back channel for communicating transmit filtercoefficient values generated by the adaptation controller to atransmitter coupled to the communications channel; wherein the signal isa baseband digital signal and the communication channel is a backplane,and the at least one threshold value is proportional to a residualpost-cursor tap coefficient value of the adaptive equalizer.
 14. Thereceiver of claim 13 wherein the limiter is a saturable amplifier.
 15. Amethod of receiving a data signal in a receiver having a linear path anda nonlinear path, comprising the steps of: recovering, from thenonlinear path, data from the received data signal; and recovering, fromthe linear path, a clock synchronized to the received data signal. 16.The method of claim 15 wherein the nonlinear path includes a limiter.17. The method of claim 16 wherein the limiter is a saturable amplifier.18. The method of claim 16 wherein the receiver includes an equalizercommon to the linear path and the nonlinear path and further comprisingthe step of: equalizing the received data signal using the equalizer.19. The method of claim 18 wherein the equalizer includes an adaptivedecision feedback equalizer response to the recovered data and therecovered clock.
 20. The method of claim 16 further comprising the stepof delaying signals in the linear path to at least partially compensatefor delay of signals by the limiter in the nonlinear path.